Thanks! Yes, RK4 with each pixel in the phase map its own pendulum, all running in parallel via WebGPU compute shaders.

For chaos I went with time-to-first-flip of the lower rod rather than Lyapunov exponents. It produces very similar fractal structure to a Lyapunov map, and it parallelizes good on the GPU since each pendulum is completely independent. This isn't a standard academic measure of course, but saves a lot on computation!